This invention relates generally to electric current measuring apparatus and is particularly directed to the measurement of the current in a beam of charged particles.
Charged particle beams are used in various fields ranging from high energy experimental physics to medical applications. Beam currents for the various applications of charged particle beams may vary from tens of amperes to pico-ampere current levels. In the latter range of current values, the current may be difficult, if not impossible, to accurately measure.
One approach currently in use for measuring particle beam currents involves the use of a flux gate magnetometer. This zero flux current transformer includes a supermalloy pick-up device which is positioned adjacent the particle beam to be measured and is magnetized by the magnetic field of the beam. An AC signal is applied to the thus magnetized supermalloy pick-up device and the extent of magnetization, which corresponds to the particle beam current intensity, is measured in terms of a second harmonic of the AC input signal. This approach, however, is of limited use at extremely low currents, e.g., in the range of nano-amperes. In addition, accurate current measurements require somewhat precise alignment of the particle beam with the magnetized supermalloy material. However, precisely locating and positioning the charged particle beam in many cases is extremely difficult, particularly where very small beam currents are involved.
The present invention is intended to overcome the aforementioned limitations of the prior art by providing a particle beam current sensor which is generally insensitive to the exact location and cross-section of a beam of charged particles in providing highly accurate beam current measurements for direct currents down to the nano-ampere range.